Disturbing Former Farmlands Can Rouse Old Pesticides, Researchers Say

Feb 01, 2006

A group of Dartmouth researchers stated on Feb. 17 they have evidence that disturbing the land where farms once thrived can mobilize both arsenic and lead that were applied as pesticides in the early 1900s. Once disturbed, these metals can then contaminate nearby surface waters.

"We continue to learn more about how past agricultural practices are affecting our current environment," said Carl Renshaw, associate professor of earth sciences at Dartmouth. "Unlike some of the pesticides used today, metals like arsenic and lead in old pesticides do not degrade over time. So the question becomes, where do they end up? As we learn more about what happens to these metals since they were applied, we can make better decisions about how to use our land."

Renshaw and his colleagues studied two New Hampshire apple orchards where the pesticide lead arsenate was once used, and they compared the data to a nearby uncontaminated field.

The researchers confirmed earlier findings that, in the former orchards, most of the arsenic and lead remains in the top ten inches of soil. The new study goes further and shows that these toxic metals do not remain in their original mineral form. Instead, they are now part of the fine silt and organic matter in the soil, which is most susceptible to erosion.

"We learned that disturbing this land, for example tilling and replanting, mobilizes the arsenic and lead," Renshaw said. "The remobilized metals were found in sediments in a stream channel that drains the tilled orchard."

Renshaw explained that it's unclear whether the metals in the sediment are taken up by plants and animals in the stream. The researchers tested the macroinvertebrate residents (midge flies and dragonflies) at the outlet of the contaminated stream, and found that, as of yet, there is no disparity in the levels of arsenic or lead.

"Historic farmlands in New Hampshire and elsewhere are increasingly being developed," Renshaw said. "While the arsenic and lead in the soils of old orchards is essentially immobile as long as the land is not disturbed, our work suggests that the development of these lands can inadvertently mobilize these metals toward bodies of water. Communities in these areas may want to ensure additional precautions are taken to control erosion when old orchard lands are disturbed in order to reduce the potential for contamination of nearby surface waters."